"Alt / Enter usually makes Media Player go to full screen, irrespective of source size. "
Not entirely true.
There is a toggle in WMP that can impede a file from not playing full screen. I ran into this the other day. To switch it go to:
Tools, Options, Performance, Advanced
On the right hand side there is a box to check/uncheck called:
Enable full-screen mode switch
Some how (or another) mine had becamed unclicked and I couldn't go full screen. Checking the click fixed the glitch (which wasn't really a glitch...but you know what I'm sayin).